Cryostat current supply

ABSTRACT

A transition conductor comprising a tubular envelope has one end dipped in a cryogenic fluid and the upper section connected to an external current source with cryogenic fluid raised in the tubular envelope during current application to the load emersed within the cryogenic fluid.

United States Patent Moisson-Franckhauser 1 Oct. 3, 1972 1 C RYOSTAT SUPPLY 5 References Cited I "2 Inventor: Francois Molsson-Franckhauser, UNITED STATES PATENTS Brefigny'sur'orge France 3 371 45 2/1968 c 11 62/5 1 anti e 14 3 Assignee. Com Gen tale DElect cite, gf e 2.756.765 7/1956 Agule ..62/218 3,358,463 12/1967 Hawkins ..62/S14 [22] Filed Sept. 30, 1970 I2 App NCL 76,926 Primary Examiner-Meyer Perlin Attorneyu e, 0 we ion, Inn acpe SghruRthl1M'Z'8tM ak l 31) Priority Data Sept France "6933345 A transition conductor comprising a tubular envelope has one end dipped in a cryogenic fluid and the upper ii 3' 25; section connected to an external current source with E Q H I I S 174/15, cryogenic fluid raised' in the tubular envelope during current application to the load emersed within the cryogenic fluid.

4 Chins, 1 Drawing Figure CRYOS'IA'I cuassnr SUPPLY BACKGROUND OF THE INVENTION I. Field of the Invention The present invention concerns a cryostat which is provided with current feeders (that is to say, parts containing the conductors which connect an external source to a load disposed in the cryogenic enclosure) which are improved in the sense that they make it possible to reduce simultaneously and to a considerable extent, the electric losses and the heat losses.

2. Description of the Prior Art The connection between a current source outside a cryostat and a load situated in the cryogenic enclosure may be made by means of normal conductors which are connected to the terminals of the load, but these conductors generate, on the one hand, considerable electric losses owing to their resistance and to the very strong currents passing through them, and on the other hand heat losses due to conduction along the conductors, one end of which extends into the cryogenic fluid, while the other end is situated outside the cryostat, that is to say, is generally at ambient temperature. The dimensions of the conductors therefore result from a compromise between the electric losses which decrease when the cross-section of the conductor increases, and

the heat losses, which increase with the cross-section of the conductor.

SUMMARY OF THE INVENTION The present invention provides, in a cryostat, current feeders in which the heat losses and the electric losses are appreciably lower than those resulting from the previously defined compromise.

The present invention relates to a cryostat which is provided with conventional insulating and circulation means, and where necessary, conventional means for recycling the cryogenic fluid. It is characterized by the fact that it comprises at least one transition conductor comprising an upper section consisting of a normally conducting material, the design of which is such as to permit circulation of the current with minimum losses when the lower end of this section is at the temperature of the cryogenic fluid while the level of the cryogenic fluid in the cryostat is in all cases below the lower end of this first section, and a superconducting or hyperconducting lower section connected at one end to the lower part of the upper section and at the other end to the electric load, and in good thermal contact with a tubular envelope, of which one open end dips into the cryogenic fluid, and in which the level of the fluid is varied by appropriate means.

In a particular embodiment, the said cryostat comprises two transition conductors for feeding the load with direct current.

In accordance with a particular embodiment, the means for varying the level of the cryogenic liquid within each of the said tubular envelopes comprises a device adapted to establish a pressure difference between the upper parts, which contain no cryogenic liquid, of the tubular envelopes on the one hand and the enclosure of the cryostat on the other hand.

BRIEF DESCRIPTION OF THE DRAWINGS The embodiment illustrated by way of non-limiting example will enable the invention to be more readily defined, with reference to the accompanying drawing, of which the single FIGURE is a diagrammatic view of a cryostat according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the single figure, the cryostat is diagrammatically represented by the contour l, the details such as the thermal insulating layers, the piping for the circulation of the cryogenic fluid, etc., not being shown. The illustrated example corresponds to the feeding, from a current source 2, of a load circuit 3 comprising a superconductive winding 4, for example that of an electromagnet, and a superconductive switch 5 connected to the terminals of the winding 4. This load circuit 3 is immersed in the cryogenic fluid 6, for example liquid helium.

Secured to the upper part of the cryostat are two tubular envelopes l0 and 11, one open end of each of which dips into the cryogenic fluid 6, while the tubular envelopes I0 and II are in thermal contact with the superconducting lower sections 16 and 17 connected to one terminal of the load circuit 3. Their upper ends, level with the cover of the cryostat, are each provided with a flow-adjusting means, for example a needle valve 24.

Cryogenic fluid or another gas is passed through a pipe 20 at a pressure whose value; is controlled by a member 21, an increase in the pressure having the ef fect of raising the level 22 in the envelopes I0 and II in relation to the level 23 in the cryostat.

The operation will be readily apparent from this description.

When it is desired to supply current to the load circuit 3, the conductors l6 and 17 must be brought into the superconducting state in order to prevent electric losses therein.

For this purpose, a pressure increase is applied, whereby the helium level in the tubular envelopes I0 is raised above that of the bath 23 as far as the connection between the superconducting sections I6, 17 and the upper sections I2, 13.

When the current applied to the load circuit 4 has reached its rated value, the superconductive switch 5 is closed and the current in the circuit 4 is constant regardless of the current supplied by the source, which may therefore be disconnected.

When the source is disconnected, i.e., when the current in the winding is in its steady state, the conductors I6 and 17 need no longer be superconducting. Therefore, in order to limit the heat losses, the level 22 is brought to the neighborhood of the level 23 by removing the additional pressure. The heat losses due to conduction will be minimum owing to the fact that the cross-section of the superconductors I6, 17 and of the tubular envelopes I0, II is a very small, due to which, these elements have very much higher thermal re sistance than the upper sections I2, 13.

Consequently, the losses in the transition conductors, which are similar to those in the conventional current feeders when they are employed with variable current, become very low in rated operation in the present case, because these conductors are at the same time electrically and thermally disconnected.

A second advantage of the present invention resides in the constancy of the total losses in the transition conductors supplied with a given current, regardless of the level 23 of the helium bath, in contrast to conventional transition conductors, in which the total losses deviate from the optimum if the helium level is not maintained substantially constant, for example by frequent transfers. The present invention thus renders possible greater independence of operation of cryostats with optimum operation of the transition conductors.

To sum up, the cryostat according to the invention has the following advantages. When the superconducting circuit, closed on itself, operates with steady cur rent, the heat losses due to conduction at the said current feeders are substantially eliminated. During operation employing the transition conductors, constancy of the total losses of the feeders with respect to the variations of the helium level, and consequently greater independence of the cryostat, are ensured.

Since the foregoing embodiment has been described only in order to illustrate the invention, any means may be replaced by similar means without departing from the scope of the invention What is claimed is:

1. In a cryostat including at least two transition conductors extending therefrom and connecting an external source to a load disposed within a cryogenic enclosure, the improvement wherein:

at least one transition conductor comprises an upper normal conductor section 14 and a connecting lower superconductor section 16 whose lower end is immersed in cryogenic liquid 6 and is connected to said cryogenic liquid immersed load 4, an imperforate tubular envelope 10 extending the length of said superconductor section 16 having its lower end immersed in said cryogenic liquid 6 being in heat conductive relationship to said superconductor section 16, and means 20 for varying the level of the cryogenic liquid 6 within said tubular envelope 10 to selectively change said superconductor section 16 from normal to superconductive.

2. The cryostat according to claim 1, wherein: both transition conductors are identical and supply direct current to the load.

3. The cryostat according to claim 1, wherein said enclosure 1 carries said cryogenic liquid 6, said transition conductor 14-16, said tubular envelope 10 and said load 4, and wherein; said cryogenic liquid 6 partially fills both said tubular envelope 10 and said enclosure 1 and said means 20 for varying the level of the cryogenic liquid within the tubular envelope 10 comprises a device which establishes a pressure difference between the upper part of the tubular envelope 10 which contains no cryogenic liquid on the one hand and the enclosure 1 on the other hand.

4. The cryostat according to claim 2, wherein said enclosure 1 carries said cryogenic liquid, said transition conductors 14-16, 15-17, said tubular envelopes 10-11, and said load 4 and wherein said cryogenic liquid partially fills both tubular envelopes 10-11 and said enclosure 1 and said me'ans 20 for varying the level of said cryogenic li ui d wit in the tubular envelopes 10- l comprlses a evlce w ich establishes a pressure difference between the upper part of said tubular envelopes 10-11 which contain no cryogenic liquid 6 on the one hand and said enclosure 1 on the other hand. 

1. In a cryostat including at least two transition conductors extending therefrom and connecting an external source to a load disposed within a cryogenic enclosure, the improvement wherein: at least one transition conductor comprises an upper normal conductor section 14 and a connecting lower superconductor section 16 whose lower end is immersed in cryogenic liquid 6 and is connected to said cryogenic liquid immersed load 4, an imperforate tubular envelope 10 extending the length of said superconductor section 16 having its lower end immersed in said cryogenic liquid 6 being in heat conductive relationship to said superconductor section 16, and means 20 for varying the level of the cryogenic liquid 6 within said tubular envelope 10 to selectively change said superconductor section 16 from normal to superconductive.
 2. The cryostat according to claim 1, wherein: both transition conductors are identical and supply direct current to the load.
 3. The cryostat according to claim 1, wherein said enclosure 1 carries said cryogenic liquid 6, said transition conductor 14-16, said tubular envelope 10 and said load 4, and wherein; said cryogenic liquid 6 partially fills both said tubular envelope 10 and said enclosure 1 and said means 20 for varying the level of the cryogenic liquid within the tubular envelope 10 comprises a device which establishes a pressure difference between the upper part of the tubular envelope 10 which contains no cryogenic liquid on the one hand and the enclosure 1 on the other hand.
 4. The cryostat according to claim 2, wherein said enclosure 1 carries said cryogenic liquid, said transition conductors 14-16, 15-17, said tubular envelopes 10-11, and said load 4 and wherein said cryogenic liquid partially fills both tubular envelopes 10-11 and said enclosure 1 and said means 20 for varying the level of said cryogenic liquid within the tubular envelopes 10-11 comprises a device which establishes a pressure difference between the upper part of said tubular envelopes 10-11 which contain no cryogenic liquid 6 on the one hand and said enclosure 1 on the other hand. 